Well, as for today, Pokemon Bank has been Re-Released in Japan (THANKS Nintendo .__.), but when Pokemon Bank was released earlier, it is possible to cheat within games like Pokemon Black/White and Black2/White2 and transfer these Pokemon to Pokemon Bank...
Therefore, I think A LOT of (especially in Japan) people are using Action Replay or so, you can pretty much cheat ANY Pokemon with this to encounter in the wild for example, and on a desired Level too, like Lv:5 or Lv:100. So it is possible to "encounter" a wild Persian of Lv:5 in " the wild", for example, catch it, and transfer it to Pokemon Bank and put it on the GTS.
So yeah, this one must be hacked/tampered with and this goes for most Legendaries too of course. Come on, all the shiny Rayquazas, Manaphy, Mew, Deoxys and so on... Those aren't obtained legitimately either, unfortunately...
So there you go, thanks for the heads up btw! :)